1. Field of the Invention
The invention relates to a return filter for fluids, in particular, hydraulic fluid.
2. The Prior Art
Such return filters (DE-22 35 246 C3) have been in use for a long time for the filtration of hydraulic fluids, in particular, hydraulic oils. Disadvantages of these return filters are, firstly, that they consist of metal and their manufacture, therefore, involves considerable effort and expense, secondly, that connection lines for the returning fluid, bore holes and elaborate, threaded inserts for reception of the return line have to be provided in the filter head, and, thirdly, that the design of the ventilation holes of these filters is complicated and expensive, as such ventilation holes have an essentially circular-cylindrical connection piece arranged in the filter head which is usually closed by a cover of larger diameter than that of the connection piece, which produces between cover and connection piece an annular gap for the passage of air therethrough. The cover is held on the connection piece by a wire bracket. It can also prove disadvantageous that in return filters used in the open air, rain water, for example, or cleaning liquids or the like can penetrate through the annular gap which is open on all sides into the ventilation filter of the return filter and cause contamination of the ventilation filter or the filtered hydraulic fluid.